1. Field of the Invention
The present invention relates to an auxiliary service channel signal transmission system, and particularly to a transmission system for auxiliary service channel signals that are extracted/inserted by digital multi-microwave transmission terminal offices and IF repeater offices.
2. Description of the Related Art
An example of such a conventional technique for an auxiliary service channel signal transmission system is described in Japanese Unexamined Patent Publication No. Hei 2-277328. In the disclosed technique, which is one transmission method for auxiliary service channel (ASC) signals, such as monitor signals for IF repeater offices and order wires, the auxiliary signal is employed to perform FM modulation on a primary signal carrier and the FM-modulated signal is transmitted to a repeater office or other terminal office.
In the conventional method, the FM-modulated wave components must be removed from a carrier before the primary signal is demodulated.
For the reception of auxiliary signals, a circuit that is employed to cancel FM-modulated wave components is provided at the reception side of a repeater office or a terminal office.
The employment of such a circuit complicates the structures of the terminal and the repeater offices, and the wave components that were FM modulated by employing the auxiliary signal cannot be sufficiently removed. Some FM-modulated wave components remain as the phase jitter of a carrier.
Although a 4-phase PSK modulation system is still applicable for the conventional system, multi-phase modulation systems, such as a 16 QAM, a 64 QAM, and a 256 QAM, are not applicable because their S/N is unsatisfactory.